1. Field of the Invention
The present invention relates to multi jackbolt tensioners and a method for producing such tensioners to improve the performance under very high compressed stresses and, more particularly to the construction of jackbolt used in such tensioners including an improved thread configuration for the jackbolts and mating threads for a stress generator component.
2. Description of the Prior Art
The present invention relates to improvements to the fastener disclosed in U.S. Pat. No. RE33,490 which is illustrated in FIG. 1 for use to clamp flanges 1 and 2 under a compressor force indicated by arrows 3 which is maintained by stress indicated by arrow 4 of a shank portion 5 adjoined with a fastener head 6 at one end and a tensioner body 7 threadedly engaged with the shank portion opposite the fastener head 6. Torque applied to jackbolts 8 threadedly engaged about the periphery of the tensioner body 7, develop a very high thrust force against a hardened washer 9. This thrust force and the opposite reaction force on fastener head 6 create a strong clamping force on the flanges. The jackbolts are subject to very high compressive stresses. Compressive stresses in excess of 200,000 psi are routinely permitted for some materials. Compressive stresses do not usually lead to failure in service, however very high torque must be applied to the jackbolts to obtain the high stresses. One way to keep the torque requirements relatively low is by the use of a suitable lubrication. Several lubricants such as graphite oil paste, are suitable to lower the torque requirement sufficiently to prevent the jackbolts from shearing.
In low temperature service the torque required to loosen the jackbolts is about the same as the torque required to tighten the jackbolts but, problems are experienced. When multi jackbolt tensioners are heated beyond the breakdown point of the oil in the graphite oil paste, the friction factor of the lubrication increases by 300 to 400%. Some synthetic oils are stable up to 400.degree. F. but above that temperature synthetic lubricates generally fail. Solids added to proprietary lubrication compounds generally fail as a lubricant above 500 to 600.degree. F.
Seizure of the jackbolt threads in the tensioner body, and jackbolts can shear under the torque required to loosen the jackbolts. A graphite lubrication compound can be reestablished by introducing a penetrating oil to penetrate along the jackbolt threads and across the compressively stressed ends of the jackbolts. Applying oil at an elevated temperature reduces the viscosity of the oil and facilitates penetration along the mating threads.
FIG. 2 illustrates a multi jackbolt tensioner where the known thread design for the mating threads of the jackbolts and the tensioner body are configured according to standard thread specifications. The jackbolts are illustrated in the state before preloading and the mating threads are in continuous contact along the length of mating engagement. The end of the jackbolt contacting the washer 9 is provided with a flat face. When the jackbolt is torqued sufficiently to establish high stress loads in the fastener, the jackbolt threads, as shown in FIG. 3, deform and upset the metal of the thread in the tensioner body 7. The thread deformation while dependent on the relative strength of the materials of the jackbolt and the tensioner body 7 have been found to generally extend sufficient convolutions identified by reference numeral 10 to a point identified by reference numeral 11 where succeeding convolutions function to transfer stress without deformation. The thread deformation causes a significant increase to the torque that must be supplied to the jackbolts in order to attain the desired clamping force which the fastener is to produce. The thread deformation also strips the deformed lower threads of lubricant which also contributes to a requirement for greater torque. It has also been found that inaccuracy in tooling used to form the mating threads produce a corresponding inaccuracy as to the site of mating engagement such as interiorly or exteriorly of the pitch diameter. When threads contact with the jackbolt threads occurs outside the pitch diameter, greater torque is necessary than when the threads bear inside the pitch diameter. In the highly stressed state the end of the jackbolt in contact with the washer 9 has been found to experience fretting which is a form of cold welding generally at the outer peripheral rim of the face surface of the jackbolt. The occurrence of fretting will occur even in the presence of lubrication when the high pressure requirements cause the lubrication compound to be hydraulically expelled from the area of mating engagement between the jackbolt and the washer.
It is an object of the invention to provide a multi jackbolt tensioner design and method for producing the same to achieve an efficient stress distribution in the mating threads of the jackbolts and the tensioner body to prevent local yielding in the lower threads which are most remote from the torque receiving ends of the jackbolts and to improve oil penetration for lowering the magnitude of torque required to stress the jackbolts for operation of the tensioner.
It is a further object of the present invention to provide a construction and arrangement of parts for a multi jackbolt tensioner to reduce the torque requirements of the jackbolts to generate a given load and to reduce the breakaway torque requirements particularly when releasing a multi jackbolt tensioner after exposure to a high temperature environment.